α-alumina (α-Al2O3) is widely used in a refractory, an abrasive material, an insulator, an electronic part, a sparkling plug, a filler, a catalyst carrier, and the like because of its excellent insulating property, heat resistance, abrasion resistance, corrosion resistance, and the like. Of those, a high-purity α-alumina is used in applications of fine ceramics, electronic parts, and the like. Conventionally, as a conventional method of producing a high-purity α-alumina, there have been used an aluminum alkoxide hydrolysis method, a tetraammonium aluminate method, an alum thermal decomposition method, an ammonium aluminum carbonate thermal decomposition method, an aluminum chloride thermal decomposition method, an underwater spark discharge method, and the like. The production processes of those methods are complicate and the raw materials to be used are expensive, resulting in extremely high cost of an α-alumina to be obtained.
Then, there are proposed some methods of producing a high-purity α-alumina by using a Bayer method which is generally employed as a method of producing a generalized α-alumina. Examples thereof include: a method involving fusing, with an electric furnace, alumina obtained by the Bayer method, giving a hollow granular material by blowing compressed air or the like to the fused alumina, and impregnating the resultant with a mineral acid to elute impurities (see Patent Document 1); a method involving pulverizing a corundum obtained by electrothermal fusing of alumina with SiO2 and washing the resultant with hydrochloric acid and hydrofluoric acid to give a high-purity α-alumina (see Patent Document 2); a method involving pulverizing aluminum hydroxide obtained by the Bayer method or a transition alumina obtained by sintering the aluminum hydroxide and burning the resultant under an atmosphere including hydrogen chloride gas at 600 to 1, 400° C. (see Patent Document 3); and a method involving covering aluminum hydroxide by the Bayer method and an intermediate alumina obtained by heat treatment of the aluminum hydroxide as raw materials with a carbonaceous substance, burning the covered product at a temperature of 800° C. or higher under reduced-pressure atmosphere, and heating the resultant under normal pressure and an oxidizing atmosphere for decarburizing (see Patent Document 4). However, with those methods, an electromelting treatment is required (Patent Documents 1 and 2) and burning under a specific atmosphere is required (Patent Documents 3 and 4), and hence it is necessary to prepare facilities other than the kiln used in producing a generalized alumina by the Bayer method.
On the other hand, when the Bayer method is employed, there is a general problem that a large amount of an Na content remains in the obtained alumina because caustic soda (sodium hydroxide) is used in the process of the Bayer method. In the case where alumina is used as an insulating material or an electronic part material, it is necessary to remove the Na content as much as possible because the Na content is a factor of inhibiting electrical insulating characteristics. From the foregoing, there are proposed: a method involving burning aluminum hydroxide in two stages, i.e. at low temperature and high temperature, and repeating washing and filtrating (see Patent Document 5); a method involving adding hydrochloric acid or aluminum chloride to aluminum hydroxide and burning the mixture in a siliceous fireproof vessel (burning vessel) (see Patent Document 6); a method involving burning aluminum hydroxide or alumina in the coexistence of hydrochloric acid or aluminum chloride, and boric acid or boron oxide (see Patent Document 7); a method involving adding hydrochloric acid or aluminum chloride to aluminum hydroxide, mixing a silica-based substance therein, burning the mixture, and thereafter, separating the silica-based substance (see Patent Document 8); and a method involving burning aluminum hydroxide to give a crystal-free amorphous alumina, washing the amorphous alumina with an aliphatic lower carboxylic acid, and burning the resultant again (see Patent Document 9). However, even with those methods, about 100 to 600 ppm of Na content is contained in the finally obtained α-alumina in any cases and the α-alumina is not necessarily used satisfactorily for applications including electronic part materials. In addition, with those methods, the effect of reducing the amounts of metals as impurity such as Si, Ca, and Fe cannot be expected.
In view of the forgoing, there are proposed the following methods: a method involving pulverizing alumina powders with an iron medium having a diameter of 2 mm or less, washing the resultant aluminum powders with hydrochloric acid or nitric acid, and further washing the resultant with sulfuric acid having a concentration of 10 N or more to remove an Fe content in alumina (see Patent Document 10); a method of reducing an Si content by burning aluminum hydroxide or alumina with addition of fluoride thereto (see Patent Document 11); and the like. However, in the method of removing an Fe content, it is necessary to use an alumina powder having a purity of 99.99 mass % or more as a starting material. In addition, the content of the impurity supposed to be Fe was reduced to only 40 to 50 ppm in examples thereof. On the other hand, in the method of reducing an Si content, the fluoride to be used also functions as a particle growth-promoting agent of α-alumina, and hence, the α-alumina to be obtained tends to become coarse particles. Therefore, the method is unsuitable for a method of producing a high-purity α-alumina for an electronic material or ceramics raw material where fine particles are demanded.    Patent Document 1: JP 55-7532 A    Patent Document 2: JP 53-79797 A    Patent Document 3: JP 08-290914 A    Patent Document 4: JP 06-37293 B    Patent Document 5: U.S. Pat. No. 2,405,275    Patent Document 6: JP 47-5744 B    Patent Document 7: JP 48-34680 B    Patent Document 8: JP 54-16398 A    Patent Document 9: JP 55-140719 A    Patent Document 10: JP 10-324519 A    Patent Document 11: JP 62-46922 A